Vacuum brake force booster with acoustic insulation
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to vacuum brake force boosters for use with automotive vehicles. More particularly, the invention relates to a vacuum brake force booster comprising a booster housing consisting of two housing sections and, through an axially movable partition, being sealingly subdivided into a vacuum chamber and a vented working chamber, and comprising a control valve mechanically actuable by a control piston, for establishing communication between the working chamber and the vacuum chamber and the atmosphere, respectively, with the axially movable control valve housing thereof being made of thermoplastic material and, in the guiding area of the control piston, comprising a plurality of cylinder segment-shaped intervals spaced from one another by means of radial reinforcing bridges. 
     2. Brief Description of the Related Art 
     A vacuum brake force booster of the aforedescribed type is known, for example, from prior published German Patent DE-OS 36 42 760. The disadvantage involved audible noise of the incoming air caused, upon actuation thereof, by the formation of eddies in the control valve area and by air incident on the booster housing sheeting, respectively. 
     It is, therefore, the object of the present invention to improve a vacuum brake force booster of the aforementioned type to the effect that the noise caused by the underlying principle of construction is substantially eliminated. 
     SUMMARY OF THE INVENTION 
     The aforementioned problems of the prior art are solved in the practice of the present invention, by lining the housing section confining the ventable working chamber and/or the intervals with an attenuating material. 
     As a result of these measures, the noise level of the in-flowing air upon actuation of the vacuum brake force booster is substantially lowered, and the resonance pattern of the control valve housing as well as the circulation of the current of air within the control valve housing, respectively, are under a positive influence. 
     Further features, advantages and fields of end-use application of the invention will become apparent from the following description of two forms of embodiment with reference to the accompanying Drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates a longitudinal section through a vacuum brake force booster according to one embodiment of the invention. 
     FIG. 2 shows the section along directional arrows A--A of FIG. 1. 
     FIG. 3 illustrates a longitudinal section through a control valve housing of another embodiment of the vacuum brake force booster in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     The vacuum brake force booster as shown in FIGS. 1 and 2 comprises a booster housing 1 formed by two interconnected housing sections 22, 23 and subdivided by an axially movable partition 2 into a working chamber 3 and a vacuum chamber 4. The axially movable partition 2 consists of a membrane plate 5 deep drawn from sheet metal, and a flexible membrane 6 in abutment therewith (not shown in any detail) and forming between the outer periphery of the membrane plate 5 and the vacuum housing 1 a rolling diaphragm serving as a sealant. 
     A control valve 9 actuable by a piston rod 8 is provided with a control piston 10 connected to the piston rod 8 and carrying a first valve seat 18, with the said control piston 10 being axially guided in a control valve housing 11 furnished with a second valve seat 19. The two valve seats 18, 19 cooperate with a poppet valve 7 which, through a prestressing sleeve 21, is forced by means of a compression spring 20 supported on the piston rod 8 against the valve seat 18. The working chamber 3, in the non-actuated position as shown, through a passageway 12 laterally extending in the control valve housing 11, is in communication with the vacuum chamber 4. 
     The brake force, through a rubber-elastic reaction member 13 accommodated in a front-sided recess of the control valve housing 11, and through a push-rod 14 comprising a top flange 15, is transmitted to an actuating piston of a master cylinder (not shown) of the brake system, which is provided on the vacuum-sided end of the brake force booster. 
     A restoring spring 16 schematically shown in the drawing which, on the vacuum-sided front wall of the booster housing 1, is supported on a flange, holds the movable partition 2 in the initial position as shown. Moreover, a second compression spring 24 is provided which is located between the poppet valve 7 and a shoulder formed on the piston rod 8, with the force thereof providing for a prestressing of the control piston 10 and the first valve seat 18 thereof, respectively vis-a-vis the poppet valve 7. 
     The housing section 23 confining the ventable working chamber 3, on the inner side thereof, is lined with an attenuating composition 17 that may be provided in the form of a foam coating, preferably a soft foam coating, insulating boards, an insulating lacquer coating or in the form of a self-adhesive flexible foil. The insulating boards may be made up of self-adhesive, flexible, bituminated insulating cardboard whereas an insulating lacquer coating may be a sprayable sound-insulating mass on a caoutchouc and bitumen/caoutchouc basis, respectively. The insulating boards may be retained in their illustrated positions (refer FIG. 1) by sheet metal flaps 29-32. 
     As illustrated, in particular by FIG. 2, the control valve housing 11, in the guiding area of the valve piston 10, includes intervals 26 spaced from one another by a plurality of reinforcing bridges 28 and equally filled with the attenuating composition and accommodating filter elements 25, respectively. 
     In the form of embodiment shown in FIG. 3, the reinforcing bridges 28 are shorter in axial length thereby forming an annular chamber not designated in any closer detail. The said annular chamber, in that form of embodiment, accommodates an annular filter 27 pushed across the guiding area of the control piston 10 and being axially supported on the reinforcing bridges 28.